1. Field of the Invention
The present invention relates generally to devices for connecting a tubular member with a tubular housing, and in particular to a locking ring that connects a casing hanger to a wellhead housing.
2. Background of the Invention
Generally a subsea high pressure wellhead housing has a landing shoulder within it for supporting a string of casing. The landing shoulder results in the portion of the bore below it being smaller in diameter than the bore portion above it. The reduced diameter of the lower portion of the bore limits the size of casing that can be ran through and supported in the wellhead housing. If a second casing string is required for a deeper portion of the well, it would have to pass through the first string. The diameter of the second string limits the size of tubing, pumps and the like that will be run through it. To avoid very large diameter wellhead housings, operators desire to minimize the difference between the final string of casing and the bore of the wellhead housing.
One type of subsea wellhead assembly employs a high pressure wellhead housing that has a bore does not have a significantly reduced diameter portion below a landing shoulder, and is thus considered “full bore”. This type of wellhead allows a larger diameter final string of casing than one that employs a landing shoulder.
In one prior art type, an annular recess is formed in the bore of the wellhead housing. A small shoulder is placed in the bore to serve as a locator. When it is desired to run the first string of casing, the operator runs a shoulder ring into wellhead housing bore on a running tool, lands on the locator shoulder and affixes the ring in the recess. The operator then runs the first string of casing and lands a conventional casing hanger on the shoulder ring.
In another method, a shoulder ring is installed in a retracted position in the wellhead housing, then the wellhead housing is run. Subsequently, the operator runs a tool that moves the shoulder ring from the retracted position to an operational position. Both of these methods require an extra trip into the wellhead housing, which is costly in deep water.
In another type, upper and lower shoulder rings are installed on the casing hanger. The upper shoulder ring is split, thus radially expansible, while the lower shoulder ring is solid. An upper landing shoulder is located in the recess and a lower landing shoulder is located below. The operator runs the casing and casing hanger with the upper shoulder ring in a retracted position. The lower shoulder ring lands on the lower landing shoulder and cams the upper shoulder ring outward into the upper landing shoulder in the recess. The shoulder rings share the load, thus the lower landing shoulder needs to be large enough to support a portion of the load. Since the lower landing shoulder only supports part of the load, it may be smaller in radial width than a conventional load shoulder, but it is greater in width than a shoulder that serves merely to locate.